1. Field of the Invention
The invention relates to an improved device for the seismic monitoring of an underground deposit, adapted to be installed in a production well for making measurements of very different kinds. The device of the invention is particularly suitable for checking the state of wells, monitoring flowrates in wells or else making seismic recordings which may indicate the evolution in time of the monitored production zone etc. It finds its applications particularly in the monitoring of an oil deposit.
2. Description of the Prior Art
From the U.S. Pat. No. 4,775,009, a method is known for installing a set of sensors in a well which, for the needs of petroleum fluid production, is equipped with a casing pipe sealed in the well by cementing. In this method, sensors are disposed outside the pipe. Once the latter is positioned in the well, it is sealed by injecting cement. The sensors are buried in the cement and so are efficiently coupled with the underground formations.
The sensors are connected to a surface recording laboratory, each by a separate line. The different lines are held in position by collars against the wall of the pipe. The sensors may be directly applied against the latter or else be housed in added sleeves enclosing said pipe.
The positioning of the different sensors must be integrated as much as possible in the procedure for installing the pipe. The pipe sections lowered successively into the well are relatively heavy and require powerful lifting means. To reduce the installation costs, positioning thereof takes place at a relatively rapid rate without special precautions.
The sections of the casing pipe are fairly robust for withstanding the stresses which may be applied thereto during handling. It is therefore important for the elements, which it is desired to position outside said pipe to provide seismic monitoring in depth, to be able to withstand shocks and compression against the walls of the well without failure. With standard diameters chosen for the borehole and the pipes, the annular spaces therebetween are generally reduced and so the protection of the added elements must take into account the insufficiency of free space.
The geophones used are often lowered to great depths. The signals which they produce in response to the waves picked up are weak. After propagation over transmission lines which may cover several kilometers, they are very greatly attenuated and often difficult to distinguish from the background noise. The attenuation is even greater if the transmission cables, which are embedded in the cement, have electric leaks.